Method of making chlorine dioxide



OC- 2, 1956 A. PECHUKAS METHOD 0F MAKING CHLORINE DIOXIDE a r rae/VEK United States Patent METHOD OF MAKING CHLORINE DIOXIDE Alphonse Pechnkas, Akron, Ohio, assignor, by mesne assignments, to Columbia-Southern Chemical Corporation, Pittsburgh, Pa., a corporation of Delaware Application October 12, 1950, Serial No. 189,824

8 Claims. (Cl. 23-152) This invention relates to a novel method of preparing chlorine dioxide. It is known that chlorine dioxide may be prepared by reaction of an aqueous solution containing trivalent chromium and alkali metal chlorate. In this case, the trivalent chromium serves to reduce the metal chlorate, thus evolving chlorine dioxide, and is itself oxidized to hexavalent chromium. A process of this character is described in U. S. Patent 2,376,935, granted May 29, 1945, to S. H. Persson.

In the normal practice of such process, a solution containing trivalent chromium is held at a temperature below 40 C. and sodium chlorate is added thereto, the amount of chlorate added being substantially in excess of 3 moles of chlorate per mole of trivalent chromium. Typical chlorate compounds which are used are the alkali metal chlorates, such as potassium or sodium chlorates. After this solution has been prepared, the solution is heated to an elevated temperature above 40 C., generally 80 to 100 C. and is contacted with an inert gas in order to strip evolved chlorine dioxide from the solution. Thereafter, the solution is cooled to a lower temperature, for example l to 30 C., upon which crystallization of sodium sulphate decahydrate occurs. These crystals are separated and the remaining solution is treated with a suitable reducingr agent, such as sulphur dioxide, at a temperature below 40 C., in order to reduce hexavalent chromium in the solution to trivalent state, whereupon the solution is recycled.

One dithculty which arises in performance of this process is due to the fact that a large volume of solution must be heated and cooled. This is a slow and laborious process. Moreover, localized overheating in one portion of the solution tends to establish explosive concentrations of chlorine dioxide, thus creating a serious hazard.

In accordance with this invention, these difficulties may vbe avoided by heating the reaction solution with a hot inert gas which will sweep evolved chlorine dioxide from the mixture and thereby form a dilute mixture of chlorine dioxide and the gaseous diluent. Steam is a particularly advantageous gaseous diluent for this purpose. However, other condensable or non-condensable inert gaseous diluents, such as air, nitrogen, carbon dioxide, and the like may be used.

This process may be very eilectively conducted by passing steam or other hot gas into contact with the solution, preferably in countercurrent contact in a packed column, bubble-plate column, or like gas-liquid contact column. ln such process, the steam simultaneously heats the solution to reaction temperature and strips evolved chlorine dioxide therefrom substantially as rapidly as formed, and produces diluated chlorine dioxide. This is advantageous since it avoids establishment of explosive concentrations of chlorine dioxide, the hot gas automatically diluting chlorine dioxide as evolved.

The accompanying drawing conveniently illustrates a typical embodiment of the contemplated invention. As illustrated in this drawing, a suitable reacting solution may ice be prepared in a suitable vessel 1. This reacting solution may be prepared simply by mixing an alkali metal chlorate, such as sodium chlorate, with chromic sulphate in aqueous medium. In order to achieve rapid reaction, it is found advantageous 'to establish an acid content of at least 2 moles of sulphuric acid or equivalent acid in the solution. The optimum acid concentration ranges between 2 and 4 moles of acid per liter. However, lhigher acid concentrations are permissible in certain instances. It will also be understood that phosphoric acid or chromic acid or other like acid may be used which is not oxidized by chlorine and chlorine dioxide, although sulphuric acid is preferred. Preparation of this solution is normally conducted at 'a temperature below about 40 C. and if necessary, suitable cooling equipment can be provided in order to maintain this temperature.

While the process is operative using large excesses of chlorate, it is found most advantageous from the standpoint of yield and speed of reaction to use yan excess of chromic compound. That is, in the subsequent reaction of chromic ion with chlorate ion, one mole of chromic ion will react with 3 moles of -chlorate ion to yield chlorine dioxide. Thus, the lamount of chromic ion present in the solution preferably should be in excess of 1 mole to 3 moles of chlorate ion. The magnitude of this excess is capable of some variation. However, best results appear to `be obtained when a Cri*3 excessief Iabout 20 to 120 percent of theoretical is used; about 50 to 100 percent excess being optimum.

When the reaction mixture is Athus prepared, very little ing reacting solution, heating the solution to the reactingv temperature and simultaneously stripping chlorine dioxide from the solution essentially Ias rapidly as produced. The

mixture of steam. and chlorine dioxide emanates from the packed column through lline 9. Sucient steam or other.

diluent `is used -t-o heat the reaction solution and `to pro;

duce a chlorine dioxide diluent mixture which contains" at least 5, and preferably 9 or more volumes `of diluent per Volume of chlorine dioxide.

The `solution which emanates from the bottom of the packed column flows through lline 15 into :a suitable receptacle 18 in which the solution is regenerated by re' duction, for further use. Such solution comprises water,

sulphuric acid, and hexavalent chromium. 'If an excess of chlorate has been used, the solution will contain chlorate. However, if the process is conducted in the preferred manner, the solution passing through line 15 will contain no significant concentration of chlorate.

The solution is regenerated by contact with sulphur dioxide or equivalent reducing agent in order to reduce the hexavalent chromium, or at least a portion thereof, to trivalent chromium. Thus, sulphur dioxide is introduced into the solution through line 17. This reduction may be conducted at any convenient temperature at which a rapid reduction will occur. Where chlorate is absent in the reaction, no particular carerneedrbe taken to maintain the temperature at a low level. Consequently, recourse to cooling, which is necessary where an excess of chlorate has been used, is eliminated. On the other hand, where chlorate is present, the regeneration must be effected at a temperature below that at which chlorine dioxide is evolved, for example below 40 C.

Following reduction of the solution to chromic state,

3 the solution is returned through line 19 to mixer 1 where sodium 'chlorate is 'added and 'the other reactants are made up to their initial concentration.

The gaseous mixture yof steam and chlorine dioxide emanating through line 9 is ldelivered to a scrubber'25 which is' cooled by'suitable means' 'to VcondenseV the steam. The'water thus condensed vsolves the chlorine dioxide and conveys 'it to the bottom of the scrubber through line 29 to a chlorine dioxide solution storage tank 31. To avoid establishment of a vacuum ldue to dissolution or condensation `of diluent, and thus to avoid consequent local explosive concentrations of chlorine dioxide, air is introduced into the upper portion of the scrubber through line 21. Water Valso may be ydirected downwardlythrough the scrubber, 'beingintroduced through line 21 to condense the steam and dissolve chlorine dioxide. Excess air vernanates through line 23 to a suitable vent,

One seriesof'tests was conducted using an ll-section vacuum jacketed 'Brun bubble-plate column into which the feed solution was fed for production of chlorine dioxide. A feed solution reservoir was connected to the column yat the top and the outlet 'of the column at the top was vented to a condenser scrubber. This scrubber consisted of 'a 35 x 180 millimeter water-cooled vertical column packed 'with '1A inch Berle saddles, and had a water-cooled cold lfmger mounted Vabove the condenser column.

Substantially 'saturated steam was introduced into the bottom ofthe column and feed solution into the `top thereof at the rates stated in the table below. The steamchlorine dioxide mixture was condensed in the scrubber by passing countercurrently to a downwardly flowing stream of water. This water was introduced into the upper Aportion of the condenser and allowed t'o trickle downwardly over .the packing, and the resulting chlorine dioxide solution was Vcollected from the bottom of the condenser. The following table tabulates the conditions osevera'l runs, and Vthe results obtained:

Components of Feed Solution Solution ClOin Y 'Y f Feed Steam 'solution rate, rate, from Run Cr-Hl, HQSOt, .NaCla, mlliliters condensmoles moles 'moles Percent liters per er,

per per per of Cr+3 per minute .grams liter liter liter Excess minute per liter A." "1.4 `2. 0 1251 1U() `2. O '3. 6 "19. 4 B 0.98 4.0 2.'1 40 1.3 `,8.2 f. 5.16.9 C 1.` '0 2. 1 .5D 2. 5 9. 4 D; 1.140 4. 0 12; 1 '100 1.'0 -3. 7

*Rate of evolution of chlorine dioxide, 0.22 mole per hour.

'"Rate of evolntionof chlorine dioxide, 0.08 mole perhour.

Although the lpresent `invention has been described as ,to .specific details of `certain embodiments thereof, it is Anot intended that such details 'shall be `regarded as limitations upon the scope-ofthe invention except insofar as Yincluded in the accompanying claims.

Whatis claimed: v

'1. The ,process of preparing chlorine Vdioxide `which comprises contactinganacid solution containing trivalent chromium and chlorate dissolved therein, with a hot'inert gas in amount sufficient to heat the solution with consequent Vevolution of chlorine :dioxide and Yto strip the evolved chlorine dioxide from the .solution vand lto .produce -a chlorine dioxide-inert gas mixture containing sucient inert gas to render themixture non-explosive.

2. The process of preparing 'chlorine dioxide which comprises forming an vaqueous acid lsolution containing trivalent chromium and chlorate dissolved therein; introducing said solution into the upper portion of a gas-liquid Contact column; introducing a hot inert gaseous diluent into the bottom of a column at a rate suicient to heat the solution to a temperature above 69 C. with consequent evolution of chlorine dioxide and to strip evolved chlorine dioxide from the solution and to produce a mixture of chlorine dioxide and diluent containing suiilcient diluent to .render the mixture non-explosive; -and removing the resultant mixture of the 'dilue'nt and the chlorine dioxide from the column.

3. The process vof claim 2 wherein the amount of diluent is sucient to produce a mixture of chlorine dioxideand diluent containing at least 9 volumes of diluent per volume of chlorine dioxide.

4. The process of claim 2 `wherein the amount of diluent used is sulcient to ensure production of a gaseous mixture containing at least 5 volumes of diluent per volume of chlorine dioxide.

5. The process of claim 2 wherein the diluent is steam.

`.6. A method yof recovering chlorine dioxide from .a solution in which chlorine dioxide has been generated which comprises'strippingthe chlorine dioxide therefrom with su'icient steam .to produce a gaseous mixture con taining in excess .of about 5 volumes ot steam per volume of chlorine dioxide, and condensing the mixture while supplying 'suflicient air to the mixture .at the zone of condensation to prevent establishment of avacuum and thus to prevent escape of chlorine dioxide from the condensing mixture.

'7. .Aprocessfo'f preparing chlorine dioxide which comprises contacting an vaqueous acid solution containing a water`soluble trivalent chromium compound and la metallic'chlorate'there'in with an amount of steam sufficient to'heat the solution to the temperature at which chlorine dioxide Vis evolved, and to strip the evolved chlorine dioxide away from the solution and producea gaseous mixture containing at least 5 volumes of steamper volurne of chlorine dioxide; the relative temperature of the steam and the solution being such that heat is supplied by Vthe Vsteam to .the solution.

8. vA methodof preparing chlorine dioxide which cornpriss heating an aqueous acid solution containing a water .solubletrivalent chromium compound and a metallic 'chlorate with an amount .of'a hot diluent gas suiicient 'to heat the solution to the temperature at which chlorine ldioxide is evolved, and to ystrip the evolved chlorinefdioxide from the solution and produce a gaseousrn'ixture containing at least 5 Vvolumes of diluentjgas per volume ofchlorine dioxide; the relative temperature ofjthe gas and the solution being such that heat is supplied VVby the gas'to -the solution.

References 'Cited in the le of this patent UNITED STATES PATENTS 

1. THE PROCESS OF PREPARING CHLORINE DIOXIDE WHICH COMPRISES CONTACTING AN ACID SOLUTION CONTAINING TRIVALENT CHROMIUM AND CHLORATE DISSOLVED THEREIN, WITH A HOT INERT GAS IN AMOUNT SUFFICIENT TO HEAT THE SOLUTION WITH CONSEQUENT EVOLUTION OF CHLORINE DIOXIDE AND TO STRIP THE EVOLVED CHLORINE DIOXIDE FROM THE SOLUTION AND TO PRODUCE A CHLORINE DIOXIDE-INERT GAS MIXTURE CONTAINING SUFFICIENT INERT GAS TO RENDER THE MIXTURE NON-EXPLOSIVE. 